A variety of battery operated devices, such as flashlights, use rechargeable batteries and provide charging circuitry which permits the batteries to be recharged without removing them from the device.
The charging circuit provides a relatively low DC current for charging the batteries from a standard AC line power source. The standard, 110-120V AC line voltage may be reduced to appropriate levels by capacitance and resistance components or by a step-down transformer. The subject circuit is particularly intended for use with step down transformers which, typically, provide significantly greater voltage drops than most capacitance-resistance devices. The AC source current is rectified to DC by a half-wave or full-wave rectifier. Half-wave rectifiers use components, generally diodes, which conduct only one half cycle of AC current to produce a "clipped", pulsating DC current output which comprises only one half cycle of the AC source. In a full-wave rectifier, a combination of oppositely biased rectifiers, generally diodes, cooperate to provide an unidirectional output which comprises both cycles of the AC current source, one half cycle having been, in effect, "inverted" to produce a direct current signal.
Charging circuits may also include indicators which signal when charging is in progress. Conventional lamps, neon tubes and light emitting diodes (LEDs) have been used as indicators. LEDs are particularly suited to this application as they consume less power than conventional lamps and as they may be readily incorporated into a charging circuit. For example, U.S. Pat. No. 4,177,413 (Ascoli, 1979) teaches a battery circuit comprising a full-wave bridge rectifier. One of the diodes in the four-diodes bridge comprises an LED. Thus, charging indication is achieved without increasing the number of components used.
If the LED is placed in series with the battery, the LED receives the full charging current. As LED's typically have a relatively small current capacity, the available charging current must be limited to the maximum LED current capacity. Accordingly, the addition of an LED in series with the battery limits the efficiency of the circuit and increases the required charging time. Moreover, as the LED provides series resistance, it increases the voltage required by the circuit to provide an adequate charging voltage across the batteries.
Even where the LED is placed in parallel with the battery and resistors are provided in series with the LED to reduce its share of the current, the LED is not independent of the battery. The parallel LED still diverts a portion of the available charging current and, so, increases the total voltage required to provide a predetermined charging current to the battery. Increased voltage requirements generally necessitate the use of larger and more expensive transformers and, so, render the circuit less economical.
Accordingly, significant advantages and economies would be achieved by providing a battery charging and indicating circuit in which charging current is not diverted from the battery to the indicating device. Further advantages and economies would be realized by providing means for deploying an indicating device in a battery charging circuit without increasing the voltage requirements of the circuit.